Attention has been drawn to an optical multilevel modulation method in aiming for constructing a super-high-speed optical transmission system of 100 Gbit/s or higher. In particular, a coherent receiving method such as DP-QPSK (Dual Polarization Quadrature Phase-Shift Keying) has attracted attention because of its advantages of enhanced optical noise immunity and compensation performance by electrical signal processing on wavelength dispersion distortion after photoelectric conversion. An application of the coherent receiver system to the transmission system has been actively studied. An optical receiver used in the coherent receiver system comprises a local oscillation light generating apparatus for generating local oscillation light, a polarization beam splitter for separating a signal light and a local oscillation light into different output ports corresponding to a polarization state, an optical 90-degree hybrid circuit for wave-combining the signal light and the local oscillation light, a photoelectric conversion section for converting an output signal from the optical 90-degree hybrid circuit into an electrical signal, an AD converter for converting the electrical signal from the photoelectric conversion section into a digital signal, and a digital signal processing (DSP) circuit for processing the digital signal. By separately detecting an in-phase component and a quadrature component of interference light of the inputted signal light and the inputted local oscillation light, it is possible to obtain information of the inputted signal light.
Among construction parts of the optical receiver used in the coherent receiver system, as to the optical 90-degree hybrid circuit, a product constituted by a spatial optical system having combined bulk type optics has been already developed and commercialized. Meanwhile, a planar light wave circuit (PLC) including optical waveguides formed on a planar substrate is superior to the above-described spatial optical system in terms of mass production capabilities and reliability. In addition, by adopting the PLC optical 90-degree hybrid circuit, for example, the feasibility in regard to integration of the polarization beam splitter and the photoelectric conversion section is increased as compared to the spatial optical system, enabling a provision of a smaller-sized optical receiver. Under these circumstances, it is expected to put the PLC optical 90-degree hybrid circuit into practice.
FIG. 1 is a construction diagram showing a conventional PLC optical 90-degree hybrid circuit. The conventional PLC optical 90-degree hybrid circuit is shown in Patent Literature 1. Patent Literature 1 relates to an optical delay interference circuit used for demodulation of a DQPSK (differential quadrature phase-shift keying) signal. This circuit itself does not correspond to the part constituting the optical receiver used in the coherent receiver system, but includes, as a part of the circuit, a function as the optical 90-degree hybrid circuit which combines two optical waves and separates the combined wave into an in-phase component and a quadrature component. Hereinafter, the in-phase component is referred to as “I component”, and the quadrature component is referred to as “Q component”. In FIG. 1, among the optical circuits described in Patent Literature 1, the construction of a circuit part alone necessary for realizing the optical 90-degree hybrid circuit is extracted to be shown.
Here, the operational principle of the conventional PLC optical 90-degree hybrid circuit shown in FIG. 1 is described. A signal light inputted from the PLC external is branched into two lights by an optical splitter 2a via an input waveguide 1a. A local oscillation light inputted from the PLC external is branched into two lights by an optical splitter 2b via an input waveguide 1b. The lights branched into two portions by the optical splitter 2a are inputted into two optical couplers 3a and 3b via arm waveguides 10a and 10b. The lights branched into two portions by the optical splitter 2b are inputted into the two optical couplers 3a and 3b via arm waveguides 10c and 10d. The signal light and the local oscillation light inputted into each of the optical coupler 3a and the optical coupler 3b are combined to be interfered with each other, which is branched into two lights for output so that a phase difference between the interference lights becomes 180 degrees. The interference lights of the signal light and the local oscillation light outputted from the optical coupler 3a travel via output waveguides 4a and 5a and are outputted into a differential optical receiver section 6a formed as an external circuit and serving as a photoelectric conversion section. The interference lights of the signal light and the local oscillation light outputted from the optical coupler 3b travel via output waveguides 4b and 5b and are outputted into a differential optical receiver section 6b formed as an external circuit and serving as a photoelectric conversion section.
A 90-degree phase shift section is provided in any one of the four arm waveguides 10a, 10b, 10c, and 10d. Thereby the interference lights outputted via the output waveguides 4a, 4b, 5a and 5b from the respective optical coupler 3a and the optical coupler 3b can be differentially demodulated by the differential optical receivers 6a and 6b to separate I component and Q component of the inputted modulation signal. Here, for simultaneously detecting I component and Q component of the modulation signal, it is necessary that waveguide lengths of the two arm waveguides 10a and 10b for transmitting the signal lights branched in the optical splitter 2a each are made equal and waveguide lengths of the two arm waveguides 10c and 10d for transmitting the local oscillation lights branched in the optical splitter 2b each are made equal excluding the 90-degree phase shift section 7. Further, waveguide lengths of the four arm waveguides 10a, 10b, 10c and 10d each are made equal excluding the 90-degree phase shift section 7, and thereby, it is possible to use this circuit also as the optical 90-degree hybrid circuit constituting the optical delay interference circuit for receiving the differential phase modulation signal such as DQPSK.
However, the problem as explained below will occur because of the construction of the 90-degree phase shift section 7. The 90-degree phase shift section 7 is installed aiming at changing an optical path length through which propagation light passes by an amount of λ×(±¼+m) only. Here, λ indicates a wave length of a signal light or a local oscillation light and m indicates an integral number. As shown in NPL 1, when a phase shift θ of the propagation light in the 90-degree phase shift section 7 is shifted out of 90 degrees, the receiving characteristic is deteriorated. For example, in a case of not correcting the shift of the phase shift θ out of 90 degrees in the digital signal processing circuit, it is necessary to control the shift of the phase shift θ out of 90 degrees within ±five degrees for restricting OSNR (optical signal noise ratio) penalty below 0.5 dB in a case of a BER (bit error rate)=10-3.
Controlling the shift of the phase shift θ out of 90 degrees in the 90-degree phase shift section 7 within ±five degrees means that it is necessary to restrict a shift of an adjustment amount in the optical path light below about 2.8% of the wavelength. For realizing such control of the phase shift θ with high accuracy, the method of controlling a phase difference between In-phase output and Quadrature output (hereinafter, called “IQ phase difference”) with high accuracy is absolutely necessary, and also it is industrially preferable that the method is simple.
FIG. 2 is the construction diagram showing a measurement method of the IQ phase difference in the conventional optical 90-degree hybrid circuit An optical delay circuit section 13 constructed of an optical splitter 11, a delay line 12 and an optical waveguide 15 is coupled to the input waveguides 1a and 1b in the conventional 90-degree hybrid circuit 8 shown in FIG. 1 for measuring the IQ phase difference. This construction aims at constructing the optical delay interference circuit by a way that instead of the signal light and the local oscillation light in FIG. 1, light outputted from the same light source is branched, one of the branched lights passes through the delay line 12 for a delay and the lights are inputted to the input waveguides 1a and 1b in the optical 90-degree hybrid circuit 8. Without mentioning, the optical path length of the delay line 12 is designed to be different from that of the optical waveguide 15.
As shown in Patent Literature 1, in a case where a difference in the optical path length between the delay line 12 and the optical waveguide 15, that is, the delay amount corresponds to one symbol amount of the signal modulated by the DQPSK method, the circuit construction shown in FIG. 2 functions as the optical delay interferometer for receiving the modulation signal of the DQPSK method. With this optical delay interference circuit, it is possible to calculate relative phase differences between lights outputted from the output waveguides 4a, 4b, 5a and 5b based upon transmission spectra outputted from the output waveguides 4a, 4b, 5a and 5b. By removing the optical delay circuit section 13 from this circuit construction after IQ phase difference evaluation is made, this circuit functions as the optical 90-degree hybrid circuit shown in FIG. 1.